(1) Field of the Invention
This invention generally relates to fluid flow measurements and more specifically to a system for providing means for accurately controlling transients in a fluid flow loop.
(2) Statement of the Prior Art
Most known water tunnels or flow loops operate under steady-state flow conditions, i.e., a constant flowrate or velocity. Several facilities have been developed in the past to provide acceleration to the flow. However, they were extremely limited in the type of accelerations or transients which they could provide to the user of the facility. One type of facility utilized a constant pressure tank which provided constant pressure to the test section. The flow could be started from rest or from some initial velocity. To initiate a transient to the flow, a valve positioned downstream of the test section would be opened as quick as possible. As the valve opened, the flow velocity would initially increase with an increasing rate of acceleration, then reach a maximum acceleration and finally the flow velocity would still increase but at an asymptotically decreasing acceleration until the acceleration became zero and the new final velocity was achieved.
A second type of transient flow control that has been used in the past is to induce sinusoidal motion onto the mean flow by incorporating some mechanical means of perturbing the flow. This can be accomplished by attaching a cylinder perpendicular to the test section itself. A piston installed in this cylinder is made to move with simple harmonic motion (i.e., it moves back and forth within the cylinder) thus taking fluid out from the test section and then pushing it back into the test section in a cyclic fashion. The flow changes from an accelerating condition to a decelerating one over each cycle of the sinusoid.
The disadvantage of both transient control systems is that the use have very little control over the type or shape of the transient time history curve that the flow undergoes. The level and duration of any one value of acceleration throughout the transient is virtually impossible to control or to be specified by the user.
To properly investigate, evaluate and test devices or products which operate under transient flow conditions, a means for controlling the flow in the test facility is thus needed.